1. Field of the Invention
The present invention relates to a ferrule for an optical connector attached to the end of an optical fiber, and more particularly to a ferrule for an optical connector formed of a resin. The present invention also relates to a metal article having a through-hole.
2. Description of the Related Art
In a communication network using optical fibers, optical connectors are used to connect optical fibers together. The optical connector requires a ferrule for concentrically butting ends of optical fibers together.
FIG. 6 illustrates a sectional view of a conventional ferrule. In FIG. 6, numeral 1 denotes a ferrule body, numeral 2 represents a through-hole formed of (i) a small diameter portion 3, (ii) a large diameter portion 4, and (iii) a tapered portion 5 expanding toward the large diameter portion 4 from the small diameter portion 3, and numeral 6 denotes a collar portion. The ferrule body 1 is formed of a resin and molded by injection molding using a metallic mold. The small-diameter-portion 3 and the large-diameter portion 4 are formed so that center axes thereof are axially aligned. In this ferrule, an optical fiber 7 is inserted, where a length of core wire 8 is inserted into the large diameter portion 4, a length of elementary wire 9 obtained by stripping the hull of the core wire is inserted into the small diameter portion 3, and the end of the elementary wire 9 is positioned at the end of the small diameter portion 3.
A conventional ferrule is formed of an epoxy resin, etc., having a higher thermal coefficient of expansion than that of the an optical fiber. This results in slippage of a butting position of glass portions of the optical fibers and increases transmission loss of optical communication.
In order to improve this problem, a double structure concentrically covering a plastic pipe body with a metallic hollow-body cylinder has been proposed as in Japanese Unexamined Patent Publication No. 58-27112.
According to this structure, it is asserted that deformation and changes in size such as contraction of the plastic material can be prevented and abrasion of the cylindrical collar due to mating and unmating operations of the plug can be well suppressed, thereby offering the advantage of sturdy connections.
Another proposed ferrule structure is formed of two concentric metallic pipes having different outer diameters in the front portion of the ferrule for connecting to a sleeve, etc., and in the back portion of the ferrule and is unitized by a resin with an optical fiber inserted therein. This structure is disclosed in Japanese Unexamined Patent Publication No. 61-137109, for example.
It is asserted that this structure has the advantages of: (i) increased bending strength because the metallic pipes having different outer diameters reinforce the mechanical strength of the resin; (ii) smaller change in size with changes in temperature (i.e., improved thermal coefficient); and (iii) preventing bending of the core wire of the optical fiber during molding of the resin.
Still another proposed structure includes a resin-made connector in which a portion of a metallic ferrule (with an end of an optical fiber cord inserted therein) is crimped and the ferrule is covered by a resin to obtain a unitarily molded item. This structure is disclosed in Japanese Unexamined Patent Publication No. 2-82206. Since the ferrule is crimped relative to the optical fiber and unitarily molded by the resin, it is asserted that the optical fiber can be prevented from separating from the connector and heat and pressure is not directly transmitted to the optical fiber (due to the metallic ferrule) during molding.
However, in the ferrule as disclosed in Japanese Unexamined Patent Publication No. 58-27112, since the elementary wire touches the plastic pipe directly and the plastic has a higher thermal coefficient of expansion, a butting position of the elementary wires of the optical fibers changes and an increase in transmission loss of optical signals results.
The ferrule according to Japanese Unexamined Patent Publication No. 61-137109 requires that the metallic pipes having different outer diameters are arranged concentrically and, therefore, positioning during the manufacturing process is difficult. Further, the connecting position of the optical fibers in the butting position of the ferrule is unstable and adversely affects the transmission loss characteristics of the device.
The ferrule disclosed in Japanese Unexamined Patent Publication No. 2-82206 is formed of a metal; in particular, brass having a large thermal coefficient of expansion is used. Disadvantageously, only crimping is employed to fix the ferrule and prevent the optical fiber from separating from the connector. Therefore, no consideration is given to changes in butting position of the elementary wires of the optical fibers, specifically due to changes in temperature.